Broadband communication systems such as cable modem systems offer users access to fast, high-bandwidth networks. A typical way of transporting data on such networks is based upon the well-known MPEG (Moving Pictures Expert Group) standard. The format of an MPEG packet header is shown in FIG. 1. Of note, is the PID (Packet ID) field, the PCR (Program Clock Reference) Flag field and PCR fields. When MPEG data is streamed in a continuous manner over a network, the resulting traffic is known as MPEG “Transport Streams”. MPEG Transport Streams are commonly used in the Cable Television industry not only for transmitting digital video and audio, but also as a “carrier” for other kinds of digital data such as that used by Cable Modems adhering for example, to the DOCSIS standard. In order to transmit these digital bitstreams, they are imposed on RF carriers by using Quadrature Amplitude Modulation (QAM). For this to work properly, it is necessary for the data rate of the MPEG stream to correspond exactly to the QAM symbol rate. “64QAM” modulation indicates that there are 6 bits for every symbol being encoded while “256QAM” modulation indicates that there are 8 bits for every encoded symbol.
Some amount of digital video presently being delivered to consumers via HFC (Hybrid Fiber Coaxial Cable) is relayed from the point of origination to multiple cable headend sites via satellite link. In order to make the physical layer conversion simple, the payload rate of the satellite channel's QPSK (Quaternary Phase Shift Keying) modulated stream is made equal to the payload rate of a 64QAM stream that is used in a cable modem facility. Although the earliest digital set-top boxes were only 64QAM-capable, later ones, including all so-called “advanced” set-top boxes, are also able to handle 256QAM. The capacity of a 64QAM channel is about 27 Mbits/second and that of a 256QAM channel is nearly 39 Mbits/second, in North America. Converting the payload stream from the lower to the higher rate would make about 12 Mbits/second available for the transmission of additional data, in the same bandwidth.
In the past, this has required one of two things: 1) the QAM rate must be slaved to the incoming MPEG rate, or 2) the MPEG rate must be slaved to the QAM rate. Neither solution is particularly attractive, and especially so in larger systems, where clock distribution may be difficult to impossible. Further, in cases where the QAM stream must be maintained even if the connection to the source of the MPEG data is interrupted, some means must be provided to devolve to a locally provided QAM clock when necessary.
Alternately, some means may be provided to “rate-adjust” the incoming MPEG stream to match a locally clocked QAM symbol rate; while this technique works, previous implementations of it have been cumbersome and expensive.